Frequency modulation detector circuits



July 3, 1945. w. VAN B. ROBERTS 2,379,748

FREQUENCY MODULATION DETECTOR CIRCUIT Filed Sept. 22, 1942 Sheets-Sheet 1 IIOICOII July 3, 1945. w. VAN B. ROBERTS 'FREQUENCY MODULATION DETECTOR CIRCUIT Filed sept.' 22, 1942 2 Sheets-Shet 2 Kul/r rs.

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Patented July 3, 1945 1 UNITEDSTATES PATENT 'OFFICE K N `irrtEQUENCY MODULATION DETCTOR CIRCUITS l Walter vanif.` "Roberts, PrincetomNN. J.; assigner to Radio Corporation of America, a corporation af Delaware v Applicationseptembezz,1942,seria1N0.4 59,z2s ,rclaims. (o1. 25o- 27) My present invention relates generallyto detectors of angular velocity-modulated carrier `Wave energy, and more particularly to frequency modulation detectors.,` M y One ofthe main objects of mypresentinven'- tion is to provide detectors of angularvelocitymodulatedcarrier wave energy, andwhich detectors are characterized by the absence yof any' frequency discriminator reactance'networks`.` but which detectors have an output balance against amplitude modulation Another important object of the invention is to provide in areceiver of frequency modulated wave energy (FM) an electron discharge device `having appreciable transit time, and which isI characterized by an output currentthat` depends upon input voltage frequency, 'therebeing derived modu- I lation voltage from theoutput of said electronic device, and the effect of "amplitude modulation of said wave `energyfleeing balanced outinfthe ab sence of frequency modulation of the input waves.

Yet another objectof myinvention is topro-` vide a receiving systemwherein a pair of inte`r` mediate frequenciespf `the -saine `value is produced, but which vary in value in opposite senses, I

` the intermediate"frequenciesbeing subjected `to amplifiers of like frequency response characteris` tics, and said characteristics being such that the amplier outputs are dependent upon the fre` quency of :input` whereby the outputsbf" the `am pliers become unequal as thefreceivcd signal departs kfrom its mean, or center, frequencyvalue.

Anotherobject of the `invention isto 'provide an FM receiving systemvwherein asingle intermediate frequency is employed, but a pair' of amplifier networks ofdifernt frequency response l characteristics `is employed whereby'detection of I the FM wave energy is secured.`

The novel featuresv which 'I believe to be char#- acteristicof my' invention are `set, forth with par# ticularity in the appended claims; the invention itself, however, as to both. its organization and method of operation willbest be understood by reference to the following" description taken in connection with the drawing in, whichV I `have indicated diagrammatically` several circuitforganizations whereby` myinvention may be. carried intoeffect.

e In the drawings;

shows a receiving system Fig. 1 schematically embodying one form of` the invention.` Fig. 2 illustrates schematically the electron discharge,` device thatmayA be employed in the .detector circuit of Fig. `1, l 1

Fig; ,3 schematically shows another embodi-` ment f on the invention,

Figs; 4 `and 5 show the respective characteristics of amplifiers A and B of Fig. 3,

Fig. 6 ,showsa further modification;

`Referringno'w to theaccompanying drawings,

wherein like reference characters in the various gures ldesignate similar circuit elements, Fig. `l.` schematicallyrepresents one form of my presentl invention. In this system thereare'provided a` pair ofindependentlocal oscillators I and 2.. For l example, oscillator I operates at 2a megacycles (me), While oscillator 2 operates at 16 mc. Each oscillator feeds an associated mixer stage.` Thus, mixer 3 is fed by oscillator I, whilemixerll is fed by oscillations from oscillator 2; `The input signal energyto leach of mixers and `I` may be either` the `originallFM signalenelgy, or, if desired, there maybe provided signal energy `produced by prior` heterodyne action. l I

, In this` case the signal collector 5 suchxaaar grounded antenna, `has applied to `it FM signal energy located inthe 42550 mc. band; the latter is the assigned FM broadcastinglband. `Alocal oscillator 6,` which isfvariable through a range of 62-70 mc., `feeds its` oscillationsyto a :mixer 1. O-f course, the inputcircuitof the mixer 'I will beK tunable with thetuning circuit of oscillatorso l that desired FM signals `can be receivedf `The intermediate frequency (I; F.) amplifier@ am-,` pliiies the resulting'` 20 mc. energy whichis fed to,` the input circuits of each of mixers 3 and 4; t

n win be noted that-thecsciuator memes` above themean, or` center, frequency of the VI.,F.

energy fed to mixer 3, while the `oscillator 2 operatesbelow the mean frequency of the energy fedtomixer 4. Hence, inthe absence of modula-` tion on the received carrier, ctirrent components ofequal frequencyare produced by the mixers 3 and ,L Those skilled in the` art are fully aware of` y the fact that in FM transmission, the carrier-rirequency al; the transmitter is varied in frequency with respectto its mean, normal, or center value in response to the modulation voltage, the frequency deviation being proportional tothe instantaneous value of said voltage.` Inlthe assigned FM band` the overall frequency` deviation has a maximum value of 1'50 kilocycles (kc.).` Of course, the fren quency values given in this application are purely l illustrative, and may be varied in accordance'with the use to-which the system is put.

fTheI'. F. `energies of mixers 3l and 4 maybe i respectively amplified in separate amplifiers.` Thus, the I. F. band pass amplifier` 9 is coupled to the` output of mixer `Bywhile,fthebandpass l from the` valuefFc.

i o said article;l

amplifier I is coupled ,to the output of mixer 4.

Each ofthe amplifiers S and Illhas its pass band designed to transmit the entire frequency deviation` range of the applied signal energy. It will be noted, as shown in Fig. 1, that one of the nor.- mally l equal intermediate frequencies increases with increase of signal `wave, frequency, vwhile the other decreases.v Inother words,` if the rinstantaneous frequency of the .applied FM signal energy has a value such that it exceeds the mean value of mc., then the I. F. energy output of mixer 3 is less than l mc., while the'I.. 'energy output of mixer tis greater than 4 mc. Hence,

it will 'be seen that the I energy-outputs of..

each of mixers 3 and 4 vary-,inv opposite m'a'gni tude senses, as the instantaneous frequency of the applied FM signal energy varies with respect to the mean, or center, frequency;

iter S is schematically represented as acting as the actual input circuit for the input electrodes of multiplier tube I5. The electron path through the multiplier I5 is represented by the dotted line; It Will be clear from the schematicrepresentation inFig. 2 that Vthe electrons are subjected toa series of multiplications, and that the outpuftfvoltageis developed across the output The circuit I6 will be non-discrim-` circuit I6. inatory, and may be the rectifier if desired. y

The falling frequencyresponse characteristic,

Vsecuredwith an electron multiplier tube of the 'type shown at I5 in Fig. 2, is caused in part by small unintentional variations in the initial veljocity of electrons from the cathode. vBy deliberatelyincreasing such variations, the frequency The I. F. energies, amplified at 9 and-Illmay i be subjected lto amplitude `limi-ting thereby to' ,l received carrier, becoming unequal as the instantaneous frequency vof. the signalfenergy deviates Accordingly," if the outputs of the devicesV II` and ,I2 bedirect current, a differential connecf tion of 'theoutput currentsto alload, suc'h as a loud speaker vor audio amplifier, vv'ill'- provide .the load with modulation voltage. f'On ythe other hand, if the outputs Aof the dfevices'lfl and I2 are in'the form of Aradio"'frequency currents, Ithese outputs maybe'sefparately rectified. I Such recti-` fiersl are schematically*designated by numerals Iand I4. 'T'Ifhe rectiiiers maybe `of any desired form, "and" it to be understood4 that" they are non-discriminatory networks. The' outputs `of these'lrectiers "areL differentially connectedfto' ythe audio frequency utilization circuit. H4Thefdevices VII and I 2 may be a pairlof identical electronl discharge deviees which act as rradiotfrequencyA amplifiers, These ampliersare preferably characterizedby arapi'dly falling oif of amplification with increasing`frequencypver afcertain range' of frequencies. 'f

In Fig. 2' r'nave scnematieny 'represented suchan amplifier device, It is 'not believed 'necessary tosliow the device in more thanschematic fashion, since' it issubstantiall'y that designated by Li'Malter inan article entitled The behavior` of electrostatic electron'multipliers as'y a function ofvfreque'ncy'. articleii'las published'in the f Proceedings pf tnefrns'tiniteof Radio' Engineers for November, 19fll.l" Reference 'is 'made to pages 592, 5Q3'and' 594'fthereof particular. The de- I mately .be reached at which the outputs of the fio vice I5 shownin'FigfZ 'is a schematic representav Ition of a multiplier device Vdesmiloed vinthe afore-` 'InFig 2 I have Vshownthe signal energy,

supplied by mixer 3 of Fig. l, which is fedthrough the amplifier 9.' Aboverthe rectanglefr'epresenting 'theamplifier 9 there isv shown the pass band characteristicthereof. It will be observedr thatthel bandris 200 kc. in Width; s0 aS efcently to transmittheentire''frequencydeviation;v A limat which rapid falling off of amplification occurs can be made, much lower. devicesof the type shown as I5 in Fig. 2 are connected in `parallel as to their input and output circuits, but operated at different electrode potentials, 'thecommon loutput can be arranged `to fall off rapidly-at a conveniently lower frequency. This is because the phase shift is greaterin the dev-ice" operating at the lower potentials, so that as frequency is increased a frequency will ultitwo devices will be in phase. opposition. At this frequency, if equal amounts of the twooutputs are/added, ,the result willbe zero. At a slightly enceis made to the characteristic shown in Fig. 14

for an 1 illustration of the. .type of characteristic which can ber-secured with suchfanarrangement. Further description` of such an arrangement will be given at a later point; The action jisexactly similar tol that l of -a pain ofv transmission lines of different lengths. When l.the `frequency `isl high enough` so that one line .is half a wave-length longer than the-- other, the output vanishesf The various-effects here described Willbe referred to-separately At first, however, it will loe-assumedthat the devices indicated at I I and I2 4of-Fig. lfare so arranged and voperated that i their amplification changes: rapidly with changing frequencies throughout afrequency 'range use of .a single device requires no further explanaj V.It/is not necessary to .use separate oscillators I and 2 since these oscillations may be wobtainedas different order harmonics ofla single oscillator.

It V,will be understood thatqcach o f. band pass ampliiiers 9 and I'may have abandpass characteristic as shownabove amplifier 9. of The rectiflers I3 and I4 are substantiallynonselective over the range of variation of signal o frequencies. These rectifiers may be of any well known type,` as for example, of thediode type; The outputs of the rectiers are differentially connected-'toV the 'primary'winding 'of an audio i output transformer. It will be seen that on For example, if two account of complete symmetry, there will be no outputdue to amplitude modulationV when :the appliedl signal energy is at its mean `value Fc. As the signal frequency changes, the frequency of one channel increases While that: of the `other ampliiication over a range Tof `frequencies.

Hence, if one of the devices `I I is adjusted to give `a falling! characteristic in the vicinity ofthe` mean frequency Fc, while the other device I2 has arising characteristic `inthe same range, the necessity for the dual channel of Fig. 1 is avoided,

and the somewhat simplerfarrangementof Fig;` 3 canbeemployed. V

Referring speciiically to Figi.` 3 the" signaly collector antenna feedsthe `mixer "I, and local oscillations are fed to theimixer from oscillator 6. In this case, the operating" I. value is 4` inc.,

and, hence, the"oscillat`o`r 6 is operatedover a range of 46 to 5eme.` An I. F.- amplifier 20 ampliiies the energy, and, after Vlimiting at ilimiter .2 I. the I. F. energy isfedto the two transmission networks shown as enclosed in'fthe` dotted? rectangles. One of these rectangles is `designated as "ampliiier A, While the other rectangle is designated as fampliiier Bf?. fIfhe characteristic shown in Fig. 4 is the frequency response characteristic of the amplifier A, while the frequency response characteristic'of- Fig. 5 is that of am i plifler B.

As explained previously, each of these ampliiiers is composed of a pair of parallel electron discharge devices such as shown at I5 in Fig. 2. In other words, it will be understoodthat the numeral 3l) in Fig. 3 corresponds to a device-such ential audio voltage output. wIt is noted that Where opposite characteristics of the twolamplifiers are obtainable with respect to their direct current outputs, the rectiflers can be omitted andthe direct` current components may be dif"` ferentially combined. I i. i f i i In Fig. 6 I have shown a further modication of the invention.

meral- 5l! designates a frequency responsive ampliiier, such as previously described, whose radio frequency output varies in phase` and/or polarity with frequency change. For example, a single `electron multiplier like I5 of` Fig.2 gives phase variation, while a pair of them, as in either dotted rectangle of Fig. 3, produces polarity reversal `as C described in connection with the output `curves of i Figs. 4 and 5. Infsuch a case, by combining the radio frequency output with a voltage derived from the input voltage by a translating; device which provides dissimilar treatment of phase or amplitude from that provided by the device 5D, `a resultant voltage is obtained which is amplitude modulated, and is, therefore, susceptible terectication. i Y i i In Fig. 6 such a system is shown` wherein balanced diode rectiers 5I andi-52 are used. `If a varying phase of the output of the device `5I) is depended upon, then the phase of thisoutputis chosen to be 'normally in quadrature with the phase of the input applied by way ofinon-discriininatory lead 53. The balanced detectors then operate in a manner quite similar to the discriminator` circuit Well knowntto the prior art. On the other hand, if thepolarity of the output reverses in accordance with the direction of the departure from the mean frequency While the amplitude increasesin proportion to said departure, then the phase of the output is chosento be colinear with that of the input over lead 53A.

While I have indicated and described several as shown by numeral I5 in Fig. 2, and that nui.

meral 3| is a similar device. 'Ihe input electrodes and output electrodes of the devices 30 and 3| are respectively in parallel. The remaining electrodes are not shown, because they are similar to that shown in Fig. 2. It will be understood that proper energization of the electrodes at devices 30 and 3l will result in the production of the substantially V-shaped characteristic of Fig. 4. The operating frequency Fc is chosen so as to be located at substantially the center ofthe falling slope to the left of the zero point of the characteristic of Fig. 4. s

In the same way, the deviceslland 4I are constructed so as to provide the response characteristic of Fig. 5. I-Iere, the value Fc is chosen so as to be located at substantially the center portion of the rising slope to the right of the zero point. However, amplifiers A and B are so relatively constructed and designed that the zero points of their response characteristics are sufliciently displaced so that the value Fc of each response characteristic is in alignment. It will now be seen that even though the I. F. energy is fed through a single channel to the two ampliiiers, the amplifiers have oppositely sloping response characteristics with respect to the center frequency of the applied signal frequency energy.-

Vsystems for carrying my invention into effect, it

will be apparent to one skilled in the art that my` invention is by no means limited to the lparticular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, 'as set forth in the appended claims.

What I claim is: 1. In a detector for angular velocity-modulated carrier wave energy, at least one electron discharge device having,appreciableelectron transit time between the electron emitter and output electrode, said device being constructed and arranged to have a'characteristic wherein output current depends upon input voltage frequency and falls sharply over a range of input frequencies, means to apply said modulated Wave energy upon the input of said device, means for derivingr modulation voltage from the output of said device, and a load circuit adapted to be supplied with said modulation Voltage.

2. In a detector for angular velocity-modulated carrier Wave energy, at least one electron discharge device having appreciable electron transit time between the electron emitter and the said device, a load circuit adapted to be supplied i with said modulation voltage.

In this modiiication, the nu- 3, Ina detector network for waves modulated infrequencya pair of electron discharge devices having input and output electrodes, each device having rappreciable transit time, and each device beingconstructed and arranged to `have a characteristic whose voutput current depends upon input voltage frequency and whichhas a ,substantially downward sloping portion, means for applying frequency modulated wave energy. of the same mean` frequency value tothe input electrodes of each of said devices, and means for deriving modulation voltage from the output electrodes of each of said devices.`

` 4; In a detector for angular velocity-modulated carrier Wave energy, atleast one electron discharge device of the multiplier type having ap.- preciable transit time, said device being constructed 'and arranged to have a' characteristic wherein output current depends upon input volt'- age frequency and decreases above a predetermined frequency, means to apply said modulated Wave energy upon the input of said device, vmeans for deriving modulation voltage from the output of said device, a load circuit adapted to be supplied with` said modulation voltage, and substantially the midpoint of the decreasing portion of the characteristic beingvat the center Afrequency of said wave energy. l

frequency'value tothe input electrodes of each of said devices, andnieans for' deriving from the output electrodes voltage. 1 v.

6. VIn combination with a source of angular velocity-modulated carrier wave energy at least two amplier tubes having'respective input and output terminals in parallel, meansl for applying'said modulated Wave energy to the input terminals of said tubes, eachof said networks having a :substantially V-shaped frequency responsercharacteristic such that the output current of each tube is y substantially zero at ia predeterminedl intermediate point of its frequency range, andthe operating frequency points of said tubes being-located'on respectively opposite lslopes of their re-` sponse characteristics. f

5. In a detector network for waves modulated y 7. In combination with a source of frequency modulatedwave energy having a predetermined center frequency value, means for deriving from said source a pair of frequency modulated wave energies Whose center frequency values are equal to the `irst mentioned value but which vary in frequency in opposite senses, at least two transmissionnetworks provided with input and output terminals, said transmission networks having identical frequency responseA characteristics, said frequency response' characteristics having the upper frequency portion thereof decreasing at 'a relatively rapid rate, `and saidv center frequency being' located at an vintermediate point of said decreasing characteristic portions, and means coupled to the output terminals of said transmission networks for 'deriving from the output lcurrents of the networks thefmodulation on said modulated wave energy.V j'

f WALTER VAN B. ROBERTS.

ofeach of said devices modulation` 

